Organoantimony and organobismuth complexes for CO₂ fixation

The utilization of organoantimony (bismuth) complexes in CO₂ fixation is reviewed in this article. The efficient synthesis of cyclic carbonates from CO₂ and epoxides over an organoantimony(v) catalyst was first reported in 1979. After that, several organoantimony(v) complexes were found to be active for CO₂ fixation. In 2009, an organoantimony(iii) complex was reported as an effective CO₂ absorbent and it is the only example of this kind. The study of organobismuth complexes for CO ₂ fixation started in 2008, and the achievements are: (i) when organobismuth oxides, hydroxides and methoxides are used as CO₂ absorbents, they are converted to carbonates, and the organobismuth carbonates can be renewed as organobismuth oxides; (ii) organobismuth complexes can be used as catalysts to transform CO₂ into epoxides as cyclic carbonates at room temperature in the presence of a co-catalyst such as LiI and Bu ₄NI; (iii) there is the development of bimetallic organobismuth complexes that show cooperative catalytic action; (iv) there is the physical fixation of CO₂ by inorganic-organic bismuth complexes; and (v) CO₂ insertion into the Bi-C bond of an organobismuth complex is disclosed. Most of the above catalytic systems can be considered as "electrophile-nucleophile". In general, the synthesis of cyclic carbonates from CO₂ and epoxides catalyzed by an organoantimony (bismuth) complex follows the mechanism: the complex first reacts with CO ₂ to form an organometallic carbonate, then the carbonate reacts with an epoxide with ring-opening by a base (e.g., n-Bu₄NI).